A laser processing apparatus has been widely used as one of machine tools for thermal treatment of metals and nonmetals, such as laser cutting, laser welding, etc. FIG. 1 shows an outline of a conventional laser processing apparatus. The laser processing apparatus 1 comprises a laser oscillator 90, a laser beam machine 3, and a numerical control device 4. A laser beam emitted from the laser oscillator 90 passes through a shading duct 5, and reaches a processing head 6 of the laser beam machine 3. The vertical position of the processing head 6, which has a condenser lens, is adjusted by means of a Z-axis movement mechanism (not shown) in response to a command from the numerical control device 4. The condenser lens of the processing head 6 converges the laser beam on a processing point of a workpiece 7, which is placed on an X-Y table of the laser beam machine 3, where the workpiece 7 is processed.
FIGS. 2a, 2b and 2c show an arrangement of a conventional laser resonator 80 which is provided in the laser oscillator 90. The laser resonator 80 is provided with a frame 9, a gas exciting device 10, and a gas cooling device 11. The frame 9, comprising front and rear aluminum plates 12 and 13 and four rods 14 which connect the front and rear plates 12 and 13, is constructed firmly lest it be easily deformable by external force. Each rod 14 is in the form of a tube made of a material such as invar in order to minimize a heat-induced dimensional change of the frame 9. While the laser resonator 80 is operating, cooling water is circulated in the rods 14. Thus, the frame 9 is designed so that its thermal deformation is extremely small.
The gas exciting device 10 comprises discharge tubes 15a and 15b arranged parallel to each other, electrodes 16a and 16b arranged on the respective peripheral walls of the discharge tubes 15a and 15b facing each other, and a high-frequency power source 16 connected to the electrodes 16a and 16b. The opposite ends of each of the two discharge tubes 15a and 15b are fixed to the front and rear plates 12 and 13 by means of discharge tube holders 20, respectively. The rear plate 13 is fitted with a turn-back block 21 having two reflectors 18 which are arranged at right angles to each other, whereby the discharge tubes 15a and 15b are connected to each other. The respective inside spaces of the discharge tubes 15a and 15b are coupled to each other by means of the block 21, thus forming one resonant space. An output mirror 17 is attached to one end of the discharge tube 15a which is situated near the front plate 12, and a rear mirror 19 is attached to one end of the discharge tube 15b which is situated near the front plate 12.
Electric power from the high-frequency power source 16 is applied to cause electric discharge between the electrodes 16a and 16b, whereby CO.sub.2 gas in each of the discharge tubes 15a and 15b is excited. Laser emitted from the excited gas is amplified as it repeatedly reciprocates in the discharge tubes between the output mirror 17 and the rear mirror 19. Part of the laser constitutes a laser beam 22, which is emitted forward (to the left of FIG. 2) from the output mirror 17.
The gas cooling device 11 is composed of a Roots blower 23, heat exchangers 24 and 25 arranged on the intake and discharge sides of the Roots blower 23 respectively, and a pipe 26. When the Roots blower 23 is activated, the gas, adjusted in temperature by means of the heat exchangers 24 and 25, circulates in the pipe 26, whereby the gas in the discharge tubes 15a and 15b is cooled.
A shutter mirror 27 is used in suspending laser processing. When the shutter mirror 27 is in the optical path, as indicated by dotted line, the laser beam 22 is caused to deviate from the main optical path for processing, and is absorbed by a beam absorber 28. A beam phase adjusting unit 29 has a phase lag reflector 30 and a zero-shift reflector 31 therein. The beam phase adjusting unit 29 serves to convert a linear polarized laser beam into a circular polarized laser beam.
In general, the laser processing is effected by converging the laser beam outputted from the laser oscillator 80. In such a case, the distance between a laser beam outlet of the laser oscillator 80 and the processing point greatly influences the laser processing performance.
FIG. 5 schematically shows the discharge tubes 15 of the laser resonator and the laser beam 22. The laser beam 22, repeatedly reflected and amplified in a section A between the rear mirror 19 and the output mirror 17 on the discharge tubes 15 and emitted through the output mirror 17, has the property of spreading as the optical path length increases. The laser processing performance, which changes depending on various factors, is largely influenced by the diameter of the laser beam 22 at the position of the condenser lens, spread angle, and intensity distribution (transverse mode), in particular. Thus, the distance (optical path length) between the output mirror 17 of the laser resonator and the processing point of the laser beam machine 3 is an important factor as it restricts the laser processing performance.
For example, in the case of laser cutting, the spread angle of the laser beam 22 is narrow in a zone B; the transverse mode is a low-order multi-mode or ring mode, as indicated by (I) or (II); and satisfactory cutting cannot be achieved due to the influence of diffraction of light emitted from the edge portion of the output mirror 17. In a zone D, the diameter of the laser beam 22 is too large. In a zone C, on the other hand, the transverse mode resembles a single mode, as indicated by (III), and the spread of the laser beam 22 is appropriate and best suited for the laser cutting. According to the result of a cutting test using a CO.sub.2 gas laser beam, the aforesaid zone C is situated within the range of 3 m to 6 m from the output mirror 17, and the distance between the output mirror 17 and the processing point obtained when the condenser lens is located within this range is an optimum optical path length.
Conventionally, in order to obtain the aforesaid optimum optical path length, a relatively long light guide distance L.sub.1 is secured between the laser oscillator 20 and the laser beam machine 3, as shown in FIGS. 1 and 3. Such a long light guide distance L.sub.1, however, places a limitation not only on the compactness of the arrangement of the laser processing apparatus 1 as the whole but also on the degree of freedom of design. Moreover, in a conventional arrangement, the beam phase adjusting unit 29 is externally attached in the manner as is shown in FIG. 3 where a circular polarized laser beam is needed at the processing point. In this arrangement, however, dust is liable to adhere to the reflectors in the unit, thereby lowering the laser processing performance.